is liquid a mixture?
Mixture: In a mixture of hydrogen and oxygen, molecules of both the gases will exist independently. You can find the molecules of H2 as well as O2. Both the gases will have their individual identities. Hydrogen will have higher dissociation enthalpy. Oxygen will be very reactive.
Compound: In water as a compound, Hydrogen (1s1) gets one electron from oxygen from the shared pair and becomes 1s2 like Helium. Oxygen (1s2 2s2 2p4) on the other hand gets two electrons from hydrogen becomes 1s2 2s2 3p6 which is like Ne again a very stable configuration. In water what you will have is molecules of H20. There is still some dissolved oxygen that comes from the atmosphere.
Thus, water is compound of hydrogen and oxygen and not a mixture of these two elements.